1. Field of the Invention
The present invention relates to a magnetooptical recording and reproducing method and apparatus of a Curie point writing type in which information can be read out and overwritten by using a magnetic Kerr effect.
2. Related Background Art
A magnetooptic disc has been known as an erasable optical disk memory. The magnetooptic disk has an advantage such that high density recording, contactless recording and reproduction, and the like can be performed as compared with a magnetic recording medium using a magnetic head. To record information in such a magnetooptic disk, in general, it is necessary to erase (magnetize in one direction, that is, initialize) the recorded portion before recording.
As a recording method in which the erasure before recording is unnecessary, there have been proposed some ideas, for example, (1) recording and reproducing heads are individually provided, or (2) a continuous laser beam is irradiated with modulation of an applied magnetic field, while the recording is executed. However, the above recording methods have drawbacks such that the scale of the apparatus is large, the costs are high, the modulation cannot be performed at a high speed, and the like.
Therefore, the applicant of the present invention has already filed an applicant directed to a magnetooptic recording method which can solve the drawbacks of the recording methods mentioned above (refer to Japanese Patent Kokai 63-153752 and the like). In this recording method, hi-level recording is performed by using a magnetooptic recording medium having vertical magnetization films of a double layer structure (first and second magnetic layers), providing two levels of laser power from a recording head (at the same time, applying a bias magnetic field), and providing a magnetic field in positions other than a head portion. The binary recording bits comprise (i) a bit where the magnetizing direction of the first magnetic layer is oriented in a stable direction relative to the magnetizing direction of the second magnetic layer and (ii) a bit where the magnetizing direction of the first magnetic layer is oriented in an unstable direction relative to the magnetizing direction of the second magnetic layer.
The above recording method can be realized by a small size recording/reproducing apparatus of a relatively simple construction. The modulation also can be executed at a relatively high speed. The overwriting also can be executed in a manner similar to recording an the magnetic recording medium.
However, even the above method has the following problems.
In the magnetooptic medium having the vertical magnetization films of a double layer structure which is used in the above method, in order to allow the state of (ii) wherein the recording bit is to stably exist, an exchange connecting force between two layers must be conveniently adjusted.
However, it is very difficult to perform such an adjustment of the exchange connecting force and it is hard to manufacture a medium which can provide a practical use level.
Therefore, to solve such a problem, the present applicant also has proposed that a third magnetic layer made of a material which exhibits an in-surface (or in-plane) magnetizing direction at a room temperature and also exhibits a perpendicular magnetizing direction at an elevated temperature upon recording or the like is provided between the first and second magnetic layers in order to adjust the exchange connecting force between the first and second magnetic layers (refer to Japanese Patent Kokai 63-316343).
The above proposal will now be described further in detail. The third magnetic layer exhibits the in-surface magnetizing direction at room temperature and reduces the exchange connecting force between the first and second magnetic layers, thereby stabilizing the recording bit of the first magnetic layer. On the other hand, when temperature rises upon recording or the like, the third magnetic layer exhibits the perpendicular magnetizing direction and increases the exchange connecting force between the first and second magnetic layers from that at room temperature, thereby enabling the magnetizing direction of the second magnetic layer to be stably transferred to the first magnetic layer.
However, even in such a medium, the in-surface magnetization of the third magnetic layer at room temperature becomes unstable due to influences by the magnetizations of the first and second magnetic layers, an elevated temperature in the apparatus, and the like, and the device cannot function sufficiently. Therefore, there is a problem of stability of the recording bit of the first magnetic layer. Such an instability of the recording bit results in a large obstacle when executing stable reproduction.